Phenological influences on the albedo of prairie grassland and crop fields

Abstract

 The albedo of land surfaces is strongly affected by transient surface conditions. For a vegetated surface, albedo can change with soil moisture, fractional canopy cover, and plant phenology as well as solar zenith angle (θ_s). In this study, the relationships between albedo and plant phenology in prairie grassland and agricultural crops were examined by removing the effect of variations in θ_s. Albedos were evaluated at a constant θ_s, which was assigned to be 20° in this study. For days with a minimum θ_s larger than 20°, a polynomial function that relates albedos to θ_s was derived and applied to estimate the albedos at 20°. After the removal of θ_s variations, the observed albedos of grassland at the Konza Prairie showed a linearly decreasing trend from spring to winter due to prairie phenology. The observed albedo of a maize field showed a clear increase from seedling to peak green stage, except when drought caused a decrease in near-infrared reflectance. The observed albedos of a winter wheat field showed an uninterrupted decrease from peak green to harvesting stage.